In combustion chambers for gas turbines, it is an aim to reduce the emissions, such as nitrogen oxides NOx and/or carbon monoxide CO. The temperatures inside the combustion chamber cause high CO and NOx emissions.
In order to reduce the emissions, so-called dry low emissions (DLE) combustion systems are used which typically have a main and supplementary fuel stream where the main fuel stream burns in a premixed flame mode and the supplementary fuel stream burns in a diffusion flame mode. The supplementary fuel stream, may generate a so-called pilot flame, which may be of a pure diffusion type or to some extent premixed. This stabilizes a main flame in the combustion chamber. A very lean fuel mixture is burned in the main flame. Under normal circumstances this would cause an unstable flame which is prone to dynamics. Therefore, the DLE systems use the pilot flame. The pilot flame comprises a rich or richer fuel mixture, wherein the rich or richer pilot flame is more stable than the lean main flame and the heat and radicals produced from this hot stable pilot frame stabilizes the main flame.
Ignition of liquid system in gas turbines has always proved problematic. There are many factors involved, such as the fuel flow rate, atomization of the fuel, air assistant flow rates, location of fuel injector/ignitor as well as the aerodynamics in the area of ignition. This is particularly true for Dry Low Emissions (DLE) systems as there are pilot flow rates to balance as well.
The freedom in positioning of an ignitor to achieve reliable ignition of a liquid pilot fuel is very limited. The ignitor and a respective pilot fuel injector are separate items and their locations rely on local aerodynamics inside the burner volume of the combustor such that the injected pilot fuel spray is washed over the ignitor.
However, if the local fluid flow tends to move away from the ignitor surface, then the injected pilot fuel could be swept away from the ignitor without being ignitioned.
In particular, the aerodynamics within the combustor is designed to reach an optimum efficiency at full load running. As a consequence, the aerodynamics at the ignitor is poor in a start and lightning phase of the gas turbine.
In order to improve ignition reliability, several methods have been employed such as gas assisted ignition, the use of plasma ignitors and the use of higher power ignitors. Gas assisted ignition requires a further gas supply, e.g. from gas bottles, which can be costly to maintain. Plasma ignitors currently have short lifetimes and there is no running experience of using them in a gas turbine. Higher power ignitors burn out quickly and the service life is short.
FIG. 9 shows a conventional pilot burner device, which comprises a conventional pilot body 900. A conventional pilot surface 901 of the conventional pilot body 900 is facing an inner volume (burner volume) of the conventional combustion chamber. A conventional fuel injector 902 and a conventional ignitor unit 903 are installed within the conventional pilot body 900, such that a conventional fuel spray 904 is injectable into the inner volume. A flow direction 106 of fluids in the inner volume directs the injected conventional fuel spray 904 to the conventional ignitor unit 903.
The ignition of liquid fuel in a gas turbine is often difficult. The fuel is atomized by, for example, the aid of an air assist flow. However, this may lead to a strengthening of the jet which propels the fuel into the combustion chamber and away from the fuel ignitor. This may result in a poor ignition performance.
It may happen, that the ignition performance is so bad that gas turbine engines have to be lit with gaseous fuel first and then transferred to an operation with liquid fuel.
U.S. Pat. No. 6,151,899 A discloses a lean-burn combustor for a gas turbine which comprises a pilot body. The pilot body comprises a recess in the centre section of the pilot body into which a pilot fuel injector is installed.
EP 2 112 433 A1 discloses a mixing chamber which is installed in a combustion chamber for generating a lean premixed combustion fluid with a high swirl. The mixing chamber comprises vortex generating elements such that a desired swirl in a combustion chamber is generated.
WO 2008/071756 A1 discloses a burner for a gas turbine engine, wherein a main fuel is injected by a swirler into a combustion chamber. The swirler comprises a number of vanes arranged in a circle in order to direct a fuel injection in a desired way. The swirler is also a partitioning device which divides a flow of air along a flow slot between the vanes such that a desired injection direction of the fuel and the air inside the combustion chamber is achieved.